The present invention relates generally to the operation of fossil fuel-fired steam generator furnaces and, more particularly, to an improved method of firing a fossil fuel-fired steam generator furnace by means of proportioning the combustion air between a first zone wherein the fuel is emitted and combustion is initiated and a second zone disposed down stream thereof to control the formation of nitrogen oxides within the furnace and by selectively positioning the second zone in relationship to the outlet of the furnace to control superheat steam temperature.
In a typical steam generator, feed water is passed through the furnace walls wherein the water absorbs heat released by the combustion of a fossil fuel within the furnace. As the water flows through the furnace water wall tubes it is raised to saturation temperature and then partially evaporated to form a steam-water mixture. The steam-water mixture is then passed to a drum wherein the water is mixed with makeup water and passed through the furnace waterwalls once again. The steam separated from the water in the drum is superheated by being passed in heat exchange relationship with the gases leaving the furnace through heat exchange surface disposed downstream of the furnace outlet.
In order to yield the desired superheat steam temperature, not only the total heat absorption in the water heating circuit, the evaporative circuit, and the steam superheater be controlled, but also that the ratio of heat absorbed in the water heating on an evaporative circuit to that absorbed in the steam superheater must be control. Although the total amount of heat absorption for a given furnace design can be controlled relatively easily by controlling the amount of fuel-fired in the furnace, controlling the ratio of heat absorption between the water heating and evaporative circuits to the absorption in the steam superheater is somewhat more difficult. Various control methods have been successfully used in the past including steam desuperheating, gas recirculation and burner tilts.
In controlling steam temperature by burner tilt, the combustion zone is physically repositioned within the furnace. To increase superheat steam temperature, the amount of heat absorption in the furnace is decreased by directing the air and fuel entering the furnace upwardly towards the furnance outlet thereby raising the combustion zone within the furnace and positioning the combustion zone closer to the furnace outlet and superheater disposed downstream thereof. To decrease steam superheat steam temperature, the heat absorption in the furnace water walls is increased by directing the fuel and air emitted to the furnace downwardly away from the furnace outlet so as to lower the combustion zone within a furnace and move the combustion zone further away from the furnace outlet and the superheater disposed downstream thereof.
A problem associated with the burner tilt method of controlling steam temperature is that the burner tilt mechanism can become very complicated. This is particularly true with respect to the new low emission burners which have been recently designed for the control of a formation of nitrogen oxides during the combustion process within the furnace. Many of these low emission burners are formed of a multiplicity of concentric ducts so that the air flow being emitted with the fuel in the combustion zone can be positioned selectively about the fuel stream so as to control mixing of the fuel and air upon admission to the furnace.
Additionally, it is well known in the prior art to further control the formation of nitrogen oxides in the combustion process of a fossil fuel-fired furnace by proportioning air flow between a first zone wherein combustion is initiated and a second zone positioned downstream of a first zone and between the first zone and the furnace outlet. In this method of controlling nitrogen oxide formation, commonly referred to as two-stage combustion or overfire air combustion, a first portion of the combustion air is emitted to the first zone in the immediate vicinity to fuel to be burned in an amount less than the theoretical amount of air required for combustion of the emitted fuel, i.e. less than the stoichiometric air requirement, while the remaining combustion air, termed overfire air, is emitted to the furnace in a downstream second zone in order to attain complete combustion of any on burned fuel before the gases leave the furnace outlet.
It is accordingly an object of the present invention to provide an improved method for firing a fossil fuel-fired steam generator wherein control of steam superheat outlet temperature may be readily achieved, and further, to provide such a method wherein control of steam superheat outlet temperature may be achieved in conjunction with the control of nitrogen oxide formation within the furnace in an intergrated control process.